1. Field of the Invention
The present invention relates to a method of fabricating a compound semiconductor device, and more particularly, to a method of fabricating a compound semiconductor device which can prevent the deterioration of the characteristics of the isolation between devices and a large step difference between two devices, when more than one semiconductor device type such as an optical device and an electron device are fabricated on a common semiconductor substrate.
2. Background of the Invention
Conventional method for fabricating more than one device type such as optical device and field effect transistor, for example, high electron mobility transistor(HEMT) on a common semiconductor substrate, comprises the steps of defining an active region of the device by wet etching an epitaxial substrate for field effect device using a single nitride film, and growing epitaxial layers for optical device by a molecular beam epitaxy(MBE) method under the condition that the sidewall of the active region formed by etching is exposed.
FIGS. 1A to 1F show a conventional method for fabricating optical device and field effect device on a semiconductor devices.
Specifically, FIG. 1A shows epitaxial layers for manufacturing the field effect device(HEMT) including a semi-insulating gallium arsenide(GaAs) substrate 1, a buffer layer 2, a sub-channel 3, a channel layer 4, a spacer layer 5, a Schottky layer 6 and an n-type GaAs ohmic layer 7.
Referring to FIG. 1B, epitaxial layers for manufacturing the optical device include an AlGaAs etch-stop layer 8, a buffer layer 9, an InGaAs absorption layer 10, an AlGaAs Schottky layer 11 and a GaAs Schottky layer 12 formed in order.
As shown in FIG. 1C, an insulating film pattern 13 is formed to define an active region of the optical device, and then the epitaxial substrate for optical device is etched to form a mesa by etching the GaAs Schottky layer 12, the AlGaAs Schottky layer 11, the InGaAs absorption layer 10, the buffer layer 9 and the etch-stop layer 8 in order, so as to form the active region of the optical device, and simultaneously the epitaxial layer in which HEMT will be located is exposed.
Referring to FIG. 1D, the epitaxial layers for HEMT device is etched in the order of the ohmic layer 7, the Schottky layer 6, the spacer layer 5, the channel layer 4, the sub-channel 3 and the buffer layer 2 to form a mesa, and then the insulating film pattern 13 is removed.
Referring to FIG. IE, an ohmic electrode 14 is formed and the ohmic layer 7 is etched to form a recessed portion. And, a gate electrode 15 of HEMT is formed. FIG. 1E is a cross-sectional view taken in the direction of the gate electrode 15. Then, Schottky electrode 16 of the optical device (MSM photo diode) is formed, and an interconnection electrode 17 connecting the above two devices is formed. Thus, the optical device and the field effect device are formed on a common substrate as shown in FIG. 1F.
In the conventional method, as can be seen from FIG. 1F, the length of the interconnection electrode 17 connecting the optical device to the field effect device is so long that there are too many parasitic components, which leads the decrease of the reliability of the device. And, the mesa etching method for forming an isolation region brings about a loss of the active region. Thus, the integration of the devices is decreased. And, the large step difference between the two devices may lead cutting of the interconnection electrode 17. And, since the gate electrode 15 of the HEMT device is in contact with the channel layer 4 of the HEMT active layers, a passage of the leakage current is generated between a gate and a source or a drain, which results the problems including the deterioration of the electric characteristics.